1. Technical Field
The invention is based on a method and a regenerative filter for equalizing digitally transmitted signals in accordance with the independent claims.
2. Discussion of the Related Art
In addition to attenuation, the signal dispersion of optical signals is the main limiting criterion that influences transmission paths and bit rates in fiber optic systems. The effects of dispersion and their limitations can be compensated for by means of suitable signal processing of the electrical signals obtained. In practical application, it is necessary to embody the signal processing adaptively since the dispersion effects of the fiber change over time. As a result of the dispersion effects, for example caused by polarization mode dispersion (PMD), there are overlaps of signal components with different polarizations. The signals are chronologically blurred due to these dispersion effects and arrive in the optical receiver in a jumbled state. Nonlinear electronic filters are used for equalizing the signals in order to once more separate out the signals that arrive at the receiver overlapping one another due to dispersion effects. The published article "Adaptive Nonlinear Cancellation for High-speed Fiber Optic Systems" by Jack Winters and S. Kasturia, Journal of Lightwave Technology, Vol. 10, No. 9, July 1992, p. 971 ff. has disclosed a nonlinear electronic filter. In order to reduce the time problems with the analog regeneration in the nonlinear filter, two threshold decision elements with different thresholds are connected to each other in parallel. The results of the parallel connected threshold decision elements are combined by means of a controllable multiplexer. The embodiment represented in FIG. 7 on page 975 of the above-cited article uses two threshold decision elements, whose outputs are connected to a multiplexer. A D flip-flop and a feedback loop switch the multiplexer of the filter. Peripheral electronics determine the thresholds to be adjusted and store them by way of capacitors. The time constants of the threshold electronics are consequently fixed. With a nonlinear filter of this kind, signals can be equalized when the delays between the slow and rapid signal components vary within a time pulse.
In order to recover the signal clock pulse with which the threshold decision element is triggered, conventional clock circuits with phase-locked loops, so-called PLL circuits (phase-locked loop), can be used. However, with very intense distortions, which arise for example with a high PMD, the following problem occurs: the signal clock pulse, which is regenerated with conventional clock circuits, has an intense phase fluctuation whose magnitude is a function of the signal distortion. Therefore, with intense signal distortions, usually the clock circuit must be enlarged further by additional phase shifters, which are incorporated as adaptive regulators into the clock pulse path in order to compensate for phase fluctuations.